1. Field of the Invention
The present invention relates in general to microcomputer systems and, particularly, to microcomputer systems employing a plurality of processing units connected to provide expanded direct memory addresses and improved data processing speed.
2. Description of Prior Art
Computers have become a part of everyday life at all levels, including the so-called personal computers utilized by individuals in the home, as well as in the office. The majority of the personal computers now available utilize microprocessors built around an 8-bit central processing unit (CPU).
This kind of 8-bit central processing unit handles 16-bit addresses and 8-bit data at one time in order to process the data stream. In an attempt to increase the processing speed and also to increase the directly accessible addresses, it has been suggested to add an additional central processing unit to the 8-bit central processing unit typically found in such personal computers. The central processing unit to be added is preferably a 16-bit central processing unit. The resultant system formed by the addition of the 16-bit central processing unit is a personal computer or microcomputer that is managed by two central processing units. Two such systems that have been previously suggested are shown in FIGS. 1 and 2. In FIG. 1, originally provided 8-bit central processing unit 1 has associated with it an appropriate memory 3 and added to this central processing unit 1 is 16-bit central processing unit 2, which has associated with it memory 4. The 8-bit central processing unit 1 and 16-bit central processing unit 2 are connected to each other through appropriate signal buses by means of input/output (I/O) port 5.
In the system of FIG. 1 central processing units 1 and 2 can use or access memories 3 and 4 separately and are capable of simultaneous data processing. Moreover, by use of input/output port 5, the transfer of data between central processing units 1 and 2 is made possible.
Nevertheless, even though the system of FIG. 1 results in additional processing capability provided by the 16-bit central processing unit 2 and its associated memory 4, in this computer system when data is to be transferred between central processing unit 1 and central processing unit 2, the so-called "handshake" operation is necessary for each data byte. This repeated required handshake operation severely slows down the transfer or handling of data. Thus, while the overall computing capability is increased, the speed of operation is decreased.
Another proposed approach to expanding the capabilities of a personal computer or microcomputer is shown in FIG. 2. In this system, additional 16-bit central processing unit 2 is added to original 8-bit central processing unit 1. In this system memory 6 is selectively connected to either of central processing units 1 or 2 by multiplexer 7, which is represented as a switch in FIG. 2. While multiplexer 7 functions at a relatively high speed, the central processing units 1 and 2 are not capable of simultaneous data processing, since memory 6 can be connected to only one of the two central processing units at a time. Therefore, while high-speed data transfer between the two central processing units (the original and the newly added one) is made possible, the dual central processing system cannot be advantageously utilized because it is not capable of simultaneous data processing.